During any machining process, relative motion between the cutting tool and part must occur. In the ideal working condition, the machine tool moves to the position commanded by the machine tool controller and the machining operation commences.
The machining operation is associated with several sources of error. First, the platen carrying the cutting tool may not move to the desired position in the direction of motion due to a difference between the actual and commanded position. This difference is called linear displacement error (LDE). Second, the machine surfaces may not be completely flat, resulting in linear error motions in the two lateral directions; such errors are called horizontal and vertical flatness or straightness errors. In addition, inaccuracies in the manufacture and assembly of the components may cause unintended rotary motions about each machine axis; such rotary motions are called roll, pitch, and yaw.
More often than not, effects of these errors do not completely cancel each other out, and their net effect will generate errors in machined features. If sufficient degrees of freedom are available, all the errors can be minimized or eliminated. However, in most machine tools, the available degrees of freedom are usually limited to three. For example, in a single axis machine tool, there is only one degree of freedom in the feed direction. Therefore, only linear displacement error motions in the direction of feed can be corrected.
Pitch and yaw are the major sources of error at the cutting insert when using long tools. The pitch error can be caused by deformation of the machine structure due to gravity, geometric errors in the components and assembly of the machine tool, and thermally induced strains due to ambient temperature changes. It is not possible to compensate for pitch and yaw errors on traditional three axis machine tools unless additional rotary axes are added to the machine.
Because geometric errors are a function of the mechanical components of the machine tool, they can usually be altered by mechanical intervention. Various techniques exist for reducing the angular errors associated with a machine tool; however, they are time consuming to execute and very laborious. In the case of errors due to gravity, there is no easy method to correct for such errors on hee axis machine tools that have only one actuator per axis. Gravity induced errors are predominantly in the Y direction, and such "droop" errors have a large effect on the pitch of the Z-axis in the YZ plane.